Motor Control Flashcards
What is the difference btwn overload protection and overcurrent protection?
Overload = within normal path, damage normally thermal Overcurrent = outside normal path, damage thermal, magnetic and arcing
What is the difference btwn a schematic diagram and a wiring diagram?
Schematic shows only electrical relationships
Wiring shows electrical and physical relationship btwn components
What is electrical interlocking?
contacts placed in the control circuit to prevent energization of the forward and reverse contacts at the same time
List 2 types of thermal overload relays
Bi-metallic
Solder pot
Explain on delay
delay occurs upon energization
Explain off-delay
delay occurs upon de-energization
What is meant by the term jogging?
Energizing the motor for a very short interval
Stop buttons must always be connected in ____?
Series
Start buttons must be connected in _______ ?
Parallel
List three reasons why across the line starting is the most common method used for starting AC motors?
High starting T
Inexpensive
Simple
Why might across the line starting not always be suitable?
High starting current causing line disturbances
High starting torque damaging to some loads
List three types of reduced voltage starting methods?
Primary resistor or reactor
Autotransformer
Wye - Delta
Describe the term “open transition”
Motor momentarily disconnects from lines during transition
Describe the term “closed transition”
Motor continuously connected to lines during transition
What advantage is offered by closed transition versus open transition starting methods?
Less line disturbances with closed transition method
What advantage would reactors have over resistors for reduced voltage starting?
What disadvantage?
Reactor are more efficient
Poorer PF
What advantage is gained by the autotransformer starting method compared to the primary impedance method?
Less line current drawn for same torque
Can the wye-delta starting method be used for all 3 phase induction motors?
No, all leads must be brought out
What important consideration should be observed when selecting the OLR heaters for wye-delta starting method?
Must be sized with phase current
Sync Motor Control: Explain the purpose of the relay SCA
Shuts down circuit if motor does not accelerate within a certain amount of time or breakdown T is exceeded.
Protects the amortisseur windings from over heating
Sync Motor Control: Explain the purpose of the relay PFR
Senses voltage induced in rotor when out of step
Decides when to apply DC to the rotor for pull-in T to occur (92-97% of sync speed)
Sync Motor Control: Explain the purpose of the contactor FC
Controls contacts which connect DC and field resistor
Sync Motor Control: Explain the purpose of the reactor
Provides voltage drops for PFR to measure
Sync Motor Control: Explain the purpose of the discharge resistor
Protects against high induced voltages in field winding when DC is applied
Across the line voltage starters can result in extreme _______ and cause ______ ?
In rush currents
Brown outs
Reduced voltage starters are commonly employed for 3 phase SCIMS in the ____ to ___ range?
5hp to 200hp
Larger motors >200hp always use RVS
Primary resistance starter advantages and disadvantages?
Advantages: Simple, reduces voltage and current, smooth accel., OK for 3, 6, 9 & 12 lead motors
Disadvantages: Power consumption, inefficient, limited capacity
Primary reactance advantages and disadvantages?
Advantages similar to resistance starter but more efficient
Disadvantages - size of inductor(s) needed.
Another name for both resistance and reactance voltage reducing starters?
Primary impedance starters
Another name for an autotransformer starter?
Compensator starter
Advantages of autotransformer starters?
High T to motor, good for large motors
Multiple taps permit on-site selection
Efficient
Most common taps for an autotransformer?
50, 65 and 80
A wye-delta starter reduced the starting T and starting I to what?
1/3 or 33%
A wye delta starter can not be used with what types of motors?
3 or 9 lead
List 2 ways to removed the R from a WRIM motor controller?
Manually with a drum controller
Automatically with contactors
What are the two common methods to do auto acceleration of WRIMs?
Definate - time acceleration using time delay relays
Speed sensing relays which sense the speed of the motor
If some R is left in a WRIM rotor circuit, the motor will run with a higher _______ ?
Slip speed (reduced T)
Below ____ % speed regulation is very unstable for WRIM speed regulator method?
50%
If the out-of-step relay stops the motor, a _______ is require before restarting.
Cooling period
Why is it essential that 2 of the contactor coils in a wye-delta starter be mechanically interlocked?
to prevent a short is both contactors closed at the same time
Give 5 methods of braking an AC motor?
Friction Plugging Dynamic Regenerative Eddy Current
Describe plugging
Energizing a motor in reverse to stop it quickly
Describe Dynamic braking
Slows/stops a motor by turning it into a generator.
Energy from motor and load dissipated as heat in motor windings or external resisitors
What are the advantages of plugging?
Fastest deceleration
No friction parts
Suitable for large motors and severe duty
Disadvantages of plugging?
Needs a reversing contactor Requires a zero speed switch or time delay relay Does not work if power fails Draws large peak currents Stresses motor windings Motor is free to rotate after stopping With time delay method motor may reverse
What prevents plugging from occuring before the motor is energized?
A lock-out solenoid
Describe friction/electromechanical braking
Friction slows/stops the motor and load using a drum, band or disc brake.
Energy is dissipated as heat
List the three types of friction brakes?
Drum
Disc
Band
Most motor friction brakes are ______ applied and ______ released.
Spring
Electromagnetically
How can the braking effect be varied on a friction brake system?
Controlling the current
Advantages of friction brakes
Hold the motor from rotating when stopped
Few control components
Automatically applies if the power fails
Brake can not reverse the motor
Disadvantages of friction brakes
Friction parts require maintenance
Brakes take up space
High initial cost
Dynamic braking is also referred to as ?
DC injection braking
Advantages of dynamic braking
Wont reverse the motor if applied to long
Gives a smooth deceleration
Braking torque is easily adjusted by varying the applied DC
No friction parts = minimal maintenance
Disadvantages of dynamic braking
DC excitation required for braking
No braking effect if supply voltage removed (loss of power)
Motor can rotate after stopping
Frequent stopping can over heat the motor
Describe regenerative braking
When a motor acts as a generator and energy is recovered and fed back to the supply
Advantages of regenerative braking
No extra equipment may be required
No friction parts
Disadvantages of regenerative braking
Only effective above sync speed
Advantages of eddy current braking
The motor wont reverse if applied to long
Braking torque is easily adjusted
No friction parts
Disadvantages of regenerative braking
More expensive than plugging, dynamic or regen braking
No braking can occur if the DC field is lost
Motor can rotates after stopping
Braking effect diminishes as motor slows
Motor starters are rated in HP because?
They must safely interrupt the locked rotor current of the motor
A 3 phase SCIM can be stopped using dynamic braking which?
Applied DC to the stator after disconnecting the AC supply
Plugging on a 3 phase induction motor is accomplished by?
Reversing the phase sequence to the stator
Starter size required for a motor load depends on?
Motor duty cycle
A wye/delta starter is commonly open transition but could be closed transition by incorporating _______ into the power circuit.
Resistors
Contractors and disconnect switches must be _________ rated
Horsepower
A shading coil is
A copper ring wrapped around the core face of an AC armature, used to reduce contact chatter
What type of motor requires 2 sets of overloads?
Multi speed motors
